Mufflers are used, for example, as components of exhaust systems of motor vehicles in general, and in particular as components of exhausts systems of cars. For specific types of cars, e.g. sports cars, it is desirable that the sound exiting the exhaust system has a sportive characteristic including sound components which are experienced as having a certain “roughness” (aggressive sound). Roughness sensitivity of humans is highest for amplitude modulated sound where the frequency of the envelope is in the range of 50 Hz to 100 Hz, although also envelope frequencies outside this range may contribute to the experienced roughness, too.
It is known that many so-called “engine orders” contribute to the amplitude modulated sound emitted by the engine, with an engine order being defined as corresponding to one revolution of the crank shaft. By way of example, at 3000 rpm (revolutions per minute) of the crank shaft the first engine order is 50 Hz (i.e. the first engine order corresponds to the rotational frequency of the crank shaft). In particular the “higher engine orders” (second and higher engine orders), that is to say multiples of the first engine order are known to contribute to the amplitude modulated sound which is responsible for the experienced roughness. Therefore, to achieve a sportive sound characteristic it is important that the higher engine orders are contained in the sound exiting the exhaust system.
To preserve the higher engine orders in the sound coming from the engine and allowing these higher engine orders to exit the exhaust system to create the sportive sound, a known two-branch exhaust component comprises a conventional muffler arranged in a first branch of the two-branch exhaust component and a bypass pipe arranged in the second branch of the two-branch component. The muffler in the first branch comprises a relatively voluminous casing in the inner space of which attenuating elements may be arranged for attenuating all types of sound passing through the inner space of the muffler and thus very substantially reducing the overall noise emitted by the engine, so that the noise exiting the exhaust system is very substantially reduced. The bypass pipe in the second branch is essentially a conventional pipe which is sized and shaped to allow the higher engine orders to pass through thus forming an acoustic bypass. The two branches are re-joined again downstream of the muffler.
While this is an acceptable and well-working constructional approach for an exhaust component from the perspective of achieving a reduced overall sound level emitted from the exhaust system and at the same time providing for a sportive sound characteristic, this constructional approach suffers from some disadvantages.
A first disadvantage is that due to the comparatively large surface of the casing of the muffler, cooling of the muffler by the ambient air is much more effective than cooling of the bypass pipe which has a comparatively small surface that is exposed to the ambient air. Accordingly, thermal expansion caused by the hot exhaust gas flowing through the two branches may lead to different lengths of the two branches. Since the two-branch component is typically mounted between an upstream exhaust pipe and a downstream exhaust pipe this different thermal expansion must be compensated for to avoid a too high thermal stress in the material which may otherwise result in that the component may get broken. To compensate for the different thermal expansions, at least one flexible element (e.g. a kind of metallic bellows) is arranged in the branch of the bypass pipe. However, such flexible elements are comparatively difficult to manufacture and are costly. A further disadvantage of the described two-branch component is that the construction is rather voluminous so that mounting of the component to the chassis of a car is comparatively space-consuming. A further disadvantage of this constructional approach is that pressure losses are generated in the two Y-joints splitting the exhaust pipe to form the two branches and re-joining the two branches again.
A further known exhaust component comprises a muffler comprising a casing and a pipe extending through the casing. The pipe comprises one or more branch conduits branching off of the pipe wall. The branch conduit(s) open(s) out into the interior of the casing. While this constructional approach is more compact than the constructional approach discussed above, it suffers from the disadvantage that pipes comprising one or more branch conduits branching off of the pipe wall are comparatively difficult and expensive to manufacture. An additional disadvantage is that the cut-off frequency of the high pass filter so formed cannot easily be tuned within the desired bandwidth without departing from the range of typical dimensions of the branched pipes.
Therefore, it is an object of the invention to provide a muffler that overcomes the above-described disadvantages while at the same time allowing higher engine orders to pass through so that the sound exiting the exhaust system has a sportive sound characteristic.